Corona discharge device having a renewable target electrode



y 969 E. J. HACKMAN A 3,

CORONA DISCHARGE DEVICE HAVING A RENEWABLE TARGET ELECTRODE Filed Feb. 2, 1966 Sheet of 2 y 1, 1959 E. J. HACKMAN T L 3,453,423

CORONA DISCHARGE DEVICE HAVING A RENEWABLE TARGET ELECTRODE Filed Feb. 2, 1966 Sheet 2 of 2 l V -W/////////// ////AV/// M A g ABSTRACT OF THE DISCLOSURE A coronadischarge device including a target electrode covered by a-movable conductive film. Fibrous material is charged by passing it in the region between the renewable surface of-the target electrode and the charging electrode (e.g., ion gun); undesirable accumulation of polymeric and otherdeposits on the apparatus is reduced.

' This invention concerns a noveland useful corona discharge device for charging fibrous elements while avoiding the accumulation of residues on the electrodes.

' A corona discharge is the electrical discharge occurring in a gas surrounding a conductor when the potential gradient at a point in a non-uniform electric field exceeds the critical value for ionization causing a local self-sustained discharge. It is a phenomenon particularly associated with sharp edged electrodes and-essentially'occurs in a limited region near the electrodes, the rest of the gap carrying a so-called dark current. The magnitude of the electric 'field gradient required to produce a corona discharge is the same as that required to produce arcing; the diflference between the two being that in corona the critical value has been'reached only in a limited region of the breakdown path between electrodes. Generally, the discharge device consists of two electrodes disposed opposite one' another across a high resistance gap in a gaseous atmosphere, the distance being sufiiciently small to pro: mote corona dischargeand sufliciently large to prevent United States Patent 3,453,428 Patented July I, 1969 eliminate other difiiculties which have not heretofore been considered.

- ,In developing apparatus for use in commercial production of film-fibril sheet means have been provided for depositing an electrostatic charge on the spread-out web to prevent its collapse into a rope-like structure. The web passes from a curved bafile opposite the spinneret hole onto an electrode e.g., a flat target plate electrode with a straight trailing edge. The target plate is electrically grounded. A second electrode, e.g., an ion gun, which is a corona charging electrode, is used to deposit an electrostatic charge-on the open web as it passes along the surface of the target plate. In this process, it has now been found that residues of polymer, pigment, fibrils, or other substances have tended to collect on the target plate surface, causing a reduction in the web charging efficiency 3 with a resulting deterioration in product quality or requiring interruption of the process while the target is cleaned or replaced. This, of course, is a disadvantage for most textile or paper uses of the product. I

It is, therefore, an object of the present invention to provide an improved apparatus for charging fibrous elements in an electrostatic field whereby accumulation of polymeric and other deposits on the electrostatic apparatus is greatly reduced.

In accordance with the present invention an apparatus is provided which is adapted to receive fibrous elements continuously forwarded in a linear path of advance in a gaseous atmosphere and to apply an electrostatic charge to the elements while avoiding accumulation of residues on the apparatus comprising, in combination:

(1) A corona discharge device consisting of a first electrode having a small radius of curvature and a second electrode having a large radius of curvature, the electrodes being situated on opposite sides of the path of the fibrous elements, the distance between the electrodes being sufiiciently small to promote corona discharge and discharge. The apparatus of the'pres'ent invention is useful in'supplying a corona field particularly inapplying an electrostatic charge to fibrous'elernents. 5 Various methods have been'described in'the artfor subjecting materials of high 'dielectricstrength 'to a corona discharge. The treatment'is especially useful for improving printability and adherability'of polyethylene. In additioh coronadishcarge is very useful for applying an electrostatic charge to high dielectric materials in the form'of particulate fibers, continuous filaments or fibrous webs; The chargedeposited on the fiber permits improved control of the fiber during processing. One satisfactory apparatus for this purpose isdescribed, for example in Disa'bato and Owens, US. Patent 3,163,753, wherein one electrode comprises an ion gun he, a row of parallel needles attached to a common conductor and the other electrode comprises a round 'bar. v I

' In, the development of a flash extrusion process based on the method of Blades and White U.S.YPatent 3,081,- 519, methods have been developed for electrostatic control of fibers. Various difficulties have arisen in controlling the fine fibrillar materials which are characteristic of this process. Accordingly, improvements have been made in the corona charging techniques. A flat target plate has been substituted for theround target bar of Disabato and Owens and the ion gunhas been modified 'by inserting high megohrn resistors in series with the needle electrodes to level out ion flow from the needles to various portions of the target plate. The present inventionconcerns further improvements which have been devised to of the apparatus of the suflicently large to prevent disruptive spark discharge, (2) A mova'ble film substantially covering the second electrode and in physical contact therewith in a region between the second electrode and the path of the fibrous elements, the film comprised of at least one conductive layer of material,

(3) Means for moving the movable film, and

(4) Means for connecting the first electrode and the conductive layer of the film to a direct current source with a potential of O to about kilovolts.

The invention will be more readily understood by reference to the drawings.

FIGURE 1 is a cross sectional :side view indicating schematically the arrangement of the various elements present invention. The spinning and target plate sections are shown greatly enlarged compared to the collecting device for greater clarity. FIG-. URE 2 is a front view of the target apparatus taken along the lines 2-2 of FIGURE 1 and shown partially in cross section.

" Considering now FIGURE 1 in more detail a polymeric solution is provided continuously through a transfer line 1 to let-down orifice 2. It passes then into letdown chamber 3 at considerably lower pressure and passes then through final orifice 4 to an area of substantially atmospheric pressure. The solution which is provided from transfer line 1 is at a temperature above the boiling point of the solvent and the pressure is at least equal to the vapor pressure of the solvent, preferably much higher. As the solution passes from the final orificeto the area of atmospheric pressure, the solvent vaporizes and a plexifilamentary strand 5 is formed. The action of the expanding solvent gas and the curved surface 6 of bafiie 7 causes the plexifilamentary strand to spread into a wide Web 11. In the embodiment shown in FIGURE 1 the baffle also oscillates back and forth. The amount and direction of oscillation is governed by rotating shaft 8. The web passes from baffle 7 to upper plate 21. As the plexifilamenta-ry web leaves the baffle it tends to foil around the off-set portion 10 due to aerodynamic action;

In FIGURE 2 the plexifilamentary strand is shown as it spreads into wide web 11 from baffle 7. The web is shown as it would flow if the bafiie were kept stationary in the center position. Actually in practice the web from an oscillating baflle would distribute itself first to one side of the target plate and then to the other side. Normally the baffie traversesan arc of 0 to 45 degrees and completes 0 to 100 cycles per second. The oscillating baflle fits J.

througha yoke 52 as shown in FIGURE 2. The upper plate 21 is insulated from the baffle 7 by means of the small gap between the two parts as shown.

Referring again to FIGURE 1 a glide block 9 is-provided to seal the junction between the baffle and the spin neret orifice. .The block is attached to the battle and has a front face which is normal to the axis of rotation of the baffle. The glide block is composed of carbon-filled polyperfiuoroethylene. It serves to prevent fibers from being caught between the baffle and the spinneret and to prevent flow of solvent gas between the two parts.

The web flows from baifie 7 to upper plate 21 of target apparatus 20. It then passes along the surface of an electro-conductive film 22 which is supported by target plate 23. As the web passes over the lower edge of the target plate and exposed film it passes within the area of corona discharge between negatively charged ion gun 37 and target plate 23. At this point a negative charge is deposited on the fibers due to a heavy concentration of negative ions which are found near the surface of the target plate. (Positive ions are more prevalent in the proximity of the ion gun.)

The ion gun 37 is composed of a number of parallel charging needles 38. These are aimed at an imaginary line just upstream from the trailing edge of the target plate. Each needle is connected by means of a high resistance element 39 (for example, 600 megohms) to common conductor 50. The common conductor is connected to a source of direct current with negative potential relative to ground. The negative direct current source is shown at 51. The insulating material which encloses the ion gun may suitably be an epoxy resin. Preferably, it should be one which resists corrosive solvent gases.

A supply of conducting film is provided from roll 25. It passes through slit 26 under brush bar 31. The film is carried across target plate 23 and passes through slit 27 to take-up roll 28, which is forwarded by means of a slow speed electric motor. The brush bar 31 tends to provide a braking action for the film to hold it tightly against target plate 23. When the film is not entirely made of conductive material throughout its thickness the brush bar also serves to transfer electrostatic charges from the film surface to ground. In either case, the target apparatus is grounded through ground wire 34. The target plate 23 is aligned perfectly with sole plate 24 by means of pins 29. Sole plate 24 is constructed of electrical insulating material, preferably one such as epoxy resin which is resistant to high voltage and corrosive solvent gases. Sockets 30 are provided for the alignment pins 29 in target plate 23 and in the sole plate 24 as shown in FIGURE 2. Brush bar 31 is spring loaded and is held in place in upper plate 21 by means of a closely-fitted slot 53. The sole plate 24 is provided at the bottom of the target apparatus to allow removal of the film rolls 25 and 28. The sole plate is constructed of insulating material. Many epoxy resins are satisfactory for this purpose. The purpose of the sole plate is to provide a sharp trailing edge 60 for the target assembly. With a rounded corner at this point the plexi-' filamentary web would tend to be carried underneath the target apparatus by the gas currents instead of falling uniformly to the collecting belt 40. The sole plate is made of insulating material to reduce the electrostatic precipita- 4 tion of degraded polymer and 'othermaterials'on' the face of the plate exposed to the spinning web.

The plate is secured by means of bolt 33. The negatively charged web 11 passes from the sole plate and is collected on a moving belt 40 or other suitable collecting device. In .the' embodiment shown the belt 40- is forwarded by means of rolls 41 whichare grounded. A lay-. down plate electrode 42 is provided underneaththe nonconducting belt 40. The negatively charged: web is deposited on moving belt 40 under theinfluence of an electrostatic field promoted bylay-down plate .42. This plate is connected to a source ,of-direct current 43 having a positivepotential relative to ground. The collecting belt is forwarded by means of rolls 41 which are grounded by wire 46. Under the influenceof the electrostatic field of lay-down -plate 42 fibrous web is attachedtoward belt 40 and is-deposited there j in random, overlappi multidirectional layers. The fibrous deposit is,.then carried un; der roll 44 whichlightly,v consolidates it. A coherent film fibril .sheet- .45" is removed continuously. from the apparatus. I p

In atypical operation of the apparatus a solution of 13% linear polyethylene (density 0.98 g./cm. and 87% trichlorofluoromethane is extruded through a let-down orifice having a diameter of .026 and a passage length 'of .0255 into a let-down chamber having avolume of-.0.636 cm. The solution is provided at a temperature of 185 C. and a pressure of 1600 p.s.i.g. to the let-down orifice. It then flows through the let-down chamber to-a final ori fice having a diameter .020 and a passage length of .0255 to the surroundingatmosphere. The solvent evaporates instantaneously. as the solution passes out of the orifice into the surrounding atmosphere. A web forms and expands as it passes over the oscillating baflie and upper plate to the film surface on the target'plate. A film composed of the above-mentioned laminate of polyvinyl fluoride is passed at speed of 0.38 cm./hr. over the face of the target plate. A negative potential of 30-60 kilovolts is applied between the ion gun. andthe target plate. The negatively charged fibers are collected, on a, moving belt under the influence of a lay-down plate electrode. A charge of 55-100 kilovolts is imposed on the lay-down plate. Afyfilm-fibril sheet of uniform opacity and thickness is produced, and the build-up of residue onthe target plate is greatly reduced. The process may, therefore, be oper-. ated continuously for many hours ,without shut-down for cleaning the target apparatus. v

' A number of variations are available for the apparatus of the invention. For. example, a number of spreading means may be used with the apparatusrIn place of the bathe of FIGURE 1 a slot-shaped shroud can be placed around the spinneret hole to provide the spreadingof the web. In addition, a varietyof battles may beused. The baflle may be either stationary, oscillating, or rotating. A rotatingbaffle, i.e., one which turns continuously in one direction, may be provided with various surface con tours to promote distribution of the web beyond the target plate. The rotating bafile may be essentially a wobble plate provided with smooth lead-in curves to promote several oscillations of the web in one turn ofthe rotary battle.

The film applied from roll 25 may be composed of a number of materials. It is necessary'that the material have sufficient conductivity to act as a grounding apparatus. The film should have reasonably constantithickness to provide uniform take-up on the take-up roll The following" materials are satisfactory: stainless steel foil, alu: minu-m foil, aluminum/poly(ethylene terephalate) filni laminate, aluminum dispersion coated on poly(ethyle"ne terephalate)film, as well as other suitable materials. A

film which'is r'enewable also may be employed, i'.e the residue may be removed from the film and thejfilm 're used. For optimum operation, the filmf which is used should be able to withstand high voltage discharge and should'be able to withstand the action of solvent. gases.

A particularly satisfactory film is a laminate comprising two layers of polyvinyl fluoride. The exposed layer of this laminate contains to 40% conductive carbon black, e.g. Cobot XC-72. This layer is about 0.0002 to 0.0010 inch thick and has high conductivity (low resistivity). The second layer is a nonconductive layer which imparts strength and durability. This layer may contain 0.5 to 5% lamp black or pigment. It is 0.001 to 0.002 inch thick. The surface resistivity of the exposed side of the laminate is about 1 ohm/cm. The base layer of nonconductive polyvinyl fluoride functions as both an insulator, if desirable, and as a support for the Weaker conducting layer. It can be made very thin because it possesses high physical strength when oriented as by stretching. The concentration of carbon black on the top layer can be increased above that of other conductive polymeric films to provide increased conductivity without adversely sacrificing or compromising the strength of the total structure. In the apparatus of FIG- URES 1 and 2 the laminate of polyvinyl fluoride film is placed with the conducting layer facing the ion gun.

A wide variety of means for moving the film may be employed other than the rolls shown herein. The means may move the film continuously or periodically when a sufiicient amount of residue is present.

Similarly, a number of ways are available for mounting the upper plate, supply rolls, take-up rolls, and other features. For example, the entire assembly may be held together by means of end plates 35 with bolts such as 36. However, for operation of the target apparatus with certain types of automatic voltage regulating apparatus, it is necessary to insulate the target apparatus electrically from the rest of the apparatus. For example, if the target plate current to ground is to be measured and controlled by means of a meter relay microammeter in ground wire 34, the target apparatus must be insulated from the rest of the machinery.

The electrodes employed may be the particular types shown herein (i.e., an ion gun and a flat target plate) or any other types suitable for use in a corona discharge device adapted to apply an electrostatic charge to fibrous elements. The present invention may be used to improve these devices which encounter the collection of a residue on the electrodes. For example, the apparatus including a round target bar electrode of Disabata and Owens, U.S. Patent 3,163,752 can be adapted to be an embodiment of the present invention. However, it is important that the electrodes used be suitable to produce corona discharge by having one electrode having a small radius of curvature, i.e., about 0.001 to about 0.005 inch such as needle point or knife edge, and a second electrode having a large radius of curvature, i.e., greater than about 0.250 inch, such as a bar or flat plate.

The power source should provide direct current with a potential in the range of 0 to about 100 kilovolts to maintain optimum web change levels during the spinning operation. The term about is used to indicate an approximate upper limit, and one skilled in the art will realize that a potential somewhat above 100 kilovolts may be suitable without departing from the heart of this invention.

The present invention is useful in the corona discharge-type charging of discrete dilectric materials including synthetic organic materials such as polyethylene, polyethylene terephthalate, polyhexamethylene adipamide, the caproamides, the acrylic fibers and indeed all of the synthetics suitable to be employed in the textile art capable of holding a charge. Similarly nonsynthetic organic materials may be used. The materials may be in the form of continuous filaments, fibrillated strands, non-woven sheets, films etc. The present invention is particularly useful in charging webs of a continuous fibrillated strand described in U.S. Patent 3,081,- 519 to Blades and White.

The apparatus of the invention is especially useful for improving the efliciency of flash-spinning and production of non-woven film-fibril sheets. One particular advantage over machines which require periodic shut-down and cleaning is that the time required to reach equilibrium target plate current at start up is reduced to a few minutes. By contrast in the absence of this invention as much as four hours time is needed to reach equilibrium after cleaning of the target plate.

Many equivalent modifications of the above invention will be apparent to those skilled in the art from a reading of the above without a departure from the inventive concept. The scope of the invention is as defined in the claims.

What is claimed is:

1. An apparatus adapted to receive fibrous elements continuously forwarded in a linear path of advance in a gaseous atmosphere and to apply an electrostatic charge to the said elements while avoiding accumulation of residues on the apparatus comprising, in combination:

(1) a corona discharge device consisting of a first electrode having a small radius of curvature and a second electrode having a large radius of curvature, said electrodes being situated on opposite sides of said path of said fibrous elements, the distance between said electrodes being sufiiciently small to promote corona discharge and sufiiciently large to prevent disruptive spark discharge,

(2) a movable film substantially covering said second electrode and in physical contact therewith in a region between said second electrode and said path of the fibrous elements, said film comprised of at least one conductive layer of material, said conductive layer being connected to ground,

(3) means for moving said movable film, and

(4) a direct current source with a potential of 0 to about kilovolts connected to said first electrode.

2. The apparatus of claim 1 wherein said first electrode is an ion gun consisting of a row of parallel conducting needles fixed rigidly along a line arranged transverse to and completely across the said path of advance with the points of the said needles aimed at the'second electrode which is a fiat target plate.

3. The apparatus of claim 1 wherein said means continuously moves said movable film.

4. The apparatus of claim 1 wherein said conductive layer is an exposed surface layer facing said first electrode.

5. The apparatus of claim 4 wherein said movable film is a laminate comprising two layers of polyvinyl fluoride, said exposed surface layer of said laminate containing 15 to 40% conductive carbon black.

6. The apparatus of claim 1 wherein said movable film is made entirely of a conductive material.

7. The apparatus of claim 1 wherein said moving means is a pair of rolls at least one of which is driven.

8. The apparatus of claim 1 further comprising a brush bar contacting said movable fil-m adapted to hold said movable film against said second electrode.

9. The apparatus of claim 1 further comprising a sole plate forming a thin straight trailing edge for said second electrode aligned with said movable film to form an essentially straight-line continuation thereof.

10. The apparatus of claim 9 wherein said sole plate is made of a solvent resistant electrical insulating material.

moving References Cited UNITED STATES PATENTS 2,907,704 10/1959 Trump. 3,320,479 5/1967 Owens.

RALPH G. NILSON, Primary Examiner. S. C. SHEAR, Assistant Examiner. 

